In various industries using electrochemical reactions, e.g., widely ranging industries from the electrolysis industry to the fuel cell industry which has been developed more and more, materials for electrodes to be used require not only electroconductivity but also corrosion resistance in the presence of a liquid such as water. Typically, electrode corrosion under severe acidic or basic conditions should be avoided in seawater electrolysis and soda electrolysis. Independently, electrode corrosion by the action of carbon monoxide in raw-material hydrogen, which is obtained by a reforming reaction, should be avoided in fuel cells.
As possible solutions to these problems, there have been proposed techniques such as (1) development of alloys (e.g., a Ni—Ti alloy (PTL 1), an alloy containing a rare-earth element (PTL 2), and a stainless steel (PTL 3)); (2) plating with a noble metal element (e.g., the formation of a platinum-plated layer on a Ti alloy (PTL 4)); and (3) coating of a metal electrode with a resinous film (e.g., coating of a platinum wire with an insulating film (PTL 5)). These techniques, however, still have problems from viewpoints of factors other than corrosion resistance, such as workability and economical efficiency.
Under such circumstances, the adaptation of ceramic materials to electrode materials has been examined. Ceramics are mainly composed of oxides, carbides, nitrides, and/or borides of inorganic elements and generally excel in mechanical strength and corrosion resistance. However, ceramics, if to be used as electrode materials, should be imparted with electroconductivity by a process of some sort, because the ceramics themselves have no electroconductivity. Typically, there have been proposed a technique of allowing a rare-earth-element-containing organic carbon compound to be present at grain boundaries of aluminum nitride (PTL 6); a technique of coating a metal constituting the electrode with aluminum oxide (PTL 7); a technique of applying a thin film of oxide ceramic to a metallic electrode substrate through a sol-gel process (PTL 8); and a technique of forming an electroconducting path between ceramic particles, which electroconducting path is composed of a reductively fired product of a polymeric compound (PTL 9).